Datasheet
8
LTC1703
1703fa
regulated output voltages as low as 800mV without exter-
nal level shifting amplifiers.
The LTC1703’s synchronous switching logic transitions
automatically into Burst Mode operation, maximizing effi-
ciency with light loads. An onboard overvoltage (OV) fault
flag indicates when an OV fault has occurred. The OV flag
can be set to latch the device off when an OV fault has
occurred, or to automatically resume operation when the
fault is removed.
2-Step Conversion
“2-step” architectures use a primary regulator to convert
the input power source (batteries or AC line voltage) to an
intermediate supply voltage, often 5V. This intermediate
voltage is then converted to the low voltage, high current
supplies required by the system using a secondary regu-
lator—the LTC1703. 2-step conversion eliminates the
need for a single converter that converts a high input
voltage to a very low output voltage, often an awkward
design challenge. It also fits naturally into systems that
continue to use the 5V supply to power portions of their
circuitry, or have excess 5V capacity available as newer
circuit designs shift the current load to lower voltage
supplies.
Each regulator in a typical 2-step system maintains a
relatively low step-down ratio (5:1 or less), running at high
efficiency while maintaining a reasonable duty cycle. In
contrast, a regulator taking a single step from a high input
voltage to a 1.xV output must run at a very narrow duty
cycle, mandating trade-offs in external component values
while compromising efficiency and transient response.
The efficiency loss can exceed that of using a 2-step
solution (see the 2-Step Efficiency Calculation section and
Figure 10). Further complicating the calculation is the fact
that many systems draw a significant fraction of their total
power off the intermediate 5V supply, bypassing the low
voltage supply. 2-step solutions using the LTC1703 usu-
ally match or exceed the total system efficiency of single-
step solutions, and provide the additional benefits of
improved transient response, reduced PCB area and sim-
plified power trace routing.
2-step regulation can buy advantages in thermal manage-
ment as well. Power dissipation in the LTC1703 portion of
a 2-step circuit is lower than it would be in a typical 1-step
Table 1. VID Inputs and Corresponding Output Voltage for
Channel 1
CODE VID4 VID3 VID2 VID1 VID0 V
OUT1
00000 GND GND GND GND GND 2.00V
00001 GND GND GND GND Float 1.95V
00010 GND GND GND Float GND 1.90V
00011 GND GND GND Float Float 1.85V
00100 GND GND Float GND GND 1.80V
00101 GND GND Float GND Float 1.75V
00110 GND GND Float Float GND 1.70V
00111 GND GND Float Float Float 1.65V
01000 GND Float GND GND GND 1.60V
01001 GND Float GND GND Float 1.55V
01010 GND Float GND Float GND 1.50V
01011 GND Float GND Float Float 1.45V
01100 GND Float Float GND GND 1.40V
01101 GND Float Float GND Float 1.35V
01110 GND Float Float Float GND 1.30V
01111* GND Float Float Float Float 1.25V
CODE VID4 VID3 VID2 VID1 VID0 V
OUT1
10000 Float GND GND GND GND 1.275V
10001 Float GND GND GND Float 1.250V
10010 Float GND GND Float GND 1.225V
10011 Float GND GND Float Float 1.200V
10100 Float GND Float GND GND 1.175V
10101 Float GND Float GND Float 1.150V
10110 Float GND Float Float GND 1.125V
10111 Float GND Float Float Float 1.100V
11000 Float Float GND GND GND 1.075V
11001 Float Float GND GND Float 1.050V
11010 Float Float GND Float GND 1.025V
11011 Float Float GND Float Float 1.000V
11100 Float Float Float GND GND 0.975V
11101 Float Float Float GND Float 0.950V
11110 Float Float Float Float GND 0.925V
11111* Float Float Float Float Float 0.900V
* 01111 and 11111 are defined by Intel to signify “no CPU.” The LTC1703
will generate the output voltages shown when these codes are selected.
APPLICATIO S I FOR ATIO
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